If a structure or building is erected on foundations which overlie a faulty area such as solution cavities, soft ground etc., the structural integrity of the building is at risk. It is well-known that a solution cavity for instance will gradually work its way towards the ground surface due to the instability of the cavity roof which periodically tends to spall off in discrete blocks of rock which then fall to the floor of the cavity and over a period of time one could expect a randomly oriented rock floor through which water from underground streams usually percolates (the underground stream is the principal cause of the cavity in the first place). As the cavity approaches the ground surface the cavity roof becomes so weak that in the case of an open field a "sink hole" or "swallow hole" appears on the ground surface where the ground surface actually deflects downwards into usually a bowl-shaped region. If the solution cavity exists beneath a building it will be evident that the building will ultimately suffer and become unsafe for occupancy.
In order to determine the presence of such cavities and other anomalies beneath the ground surface it has been the practice of foundation engineers to perform a site investigation. These site investigations have been traditionally performed by the simple expedient of driving boreholes from the ground surface and if solid soil and rock are encountered to some prescribed depth, it has been assumed that the building can be safely erected. Historically however, it has been proved on many occasions that boreholing by itself is not satisfactory because the boreholing method simply samples along the line of the borehole only; i.e. the volume of ground samples is insufficient. Other techniques have utilized the boreholes in order to extend the volume of ground samples beneath a given site, these methods include radiometric methods, gamma ray, neutron backscattering, resistivity methods, gravity methods, magnetometer methods etc.: all of which are described adequately in the literature. Depending upon a number of factors including site conditions, all these methods have their inherent advantages and disadvantages. For example, in one X-ray method, an X-ray source is used down a borehole to detect objects in a matrix of differing degrees of density e.g. a boulder suspended within clay. However the penetration of the X-rays into the clay is no more than a few borehole diameters and if a borehole was driven into rock and by-passed a large cavity only a matter of two or three feet away from the cavity wall, the X-ray method (and most other methods) would not detect the presence of the cavity.
It is among the objects of this invention to provide a method for investigating the soundness of structures which is applicable inter alia to site investigation and to the investigation of the soundness of existing engineering structures at the time of investigation.
A further object of this invention is to provide a trace indicating the soundness of the structure at the time in question.
To the best of my knowledge it has not previously been possible to follow the fatigue of a structure as a whole and in particular to be able to tell if, and when, the soundness or structural integrity of the whole structure passes a point from which it will catastrophically decay. Equally, to the best of my knowledge, it has not been readily possible heretofore to follow the effect of accidental damage or the effect of environmental forces on a structure and again to tell when the effect of these phenomena has passed the point of danger.
My interest in the monitoring of the soundness or structural integrity of structures over a period of time and the realization of the value of my methods in such context arose from a consideration of the problems associated with offshore structures. Such structures present peculiar difficulties; and these difficulties are compounded by historical factors.
Historically, naval architects have experience in the design of ships and other structures which float while civil engineers usually require foundations for their structures; and yet the design of an offshore structure requires a combination of these and other disciplines in order to advance as a new technology. It could be argued that there are parallels, such as harbour works, but in general, harbour works are in shallow waters, are relatively protected from the open sea environment and can gain great strength and rigidity by backing on to a land mass.
The offshore structure is exposed to the worst of the elements, wave dynamics, wind and gust loads, shear currents, high hydrostatic pressures, operating impact and vibration loads, accidental loads from tender vessels, the marine life and associated chemical environments to name a few. These tall, relatively slender structures therefore experience shear forces, bending and twisting moments; and are responding dynamically all the time. Add to this the unforeseen or included faults, such as microcracking in concrete components, the occasional lapse in workmanship during assembly, and the stress concentrations thus produced, and the structure becomes a prime candidate for corrosion fatigue failures, brittle fractures and other equally serious failures. The unique combination of the elements and unintentional but inherent faults in the structure produces a total environment which has not previously been experienced by all the disciplines needed to design, manufacture and operate an offshore structure.
In its endeavour to exploit the natural resources underneath the North Sea as rapidly as possible, the oil and natural gas extraction industry has until now had to rely in a major part on the existing technology in offshore structures; but this existing technology is associated in the main with shallow water conditions in the far more friendly environment of the Gulf of Mexico. There is very real risk (as has recently been confirmed by misadventure reported in practice in news reports and in the technical pass) in extrapolating designs produced for such conditions for operation under the much more severe conditions of the North Sea; and I envisage that designs for offshore structures constructed in accordance with this invention will find an important place where the monitoring of the day to day performance of such offshore structures is desired in more northern climes, particularly under exposed or deep water operation, so that the first signs of incipient failure can be detected at an early enough stage to permit remedial work to be performed, and also so that we can learn how such structures will operate under more severe conditions, to incorporate the lessons learned in the next generation of offshore structures. In addition, there is an evident need for the development of a technology in deep water offshore structures.
It is accordingly among the objects of this invention to provide methods of and apparatus for monitoring the soundness or structural integrity and performance of various structures over a period of time.
It is also an object of this invention to provide methods for the continuous monitoring of the soundness or structural integrity of offshore structures at all stages in the development of the technology of deep water offshore structures from tests on models, through the prototype stages, to continuous monitoring of operation structures.